Automotive air conditioning compressors are typically powered from the engine through a belt driven pulley. Power is indirectly applied through an electromagnetic clutch that turns on and off in response to cooling demand, which is more efficient than running the compressor one-to-one with the engine at all times. The pulley is supported on the compressor housing for free rotation about the shaft on a bearing that fixes it axially relative to the shaft. An annular friction disk fixed to the pulley axially faces an annular magnetic armature with an axial gap therebetween. A magnetic coil on the housing, located close to the friction disk and armature, pulls them together with an axial stroke sufficient to close the gap so that the pulley rotation is translated to the armature.
How the torque translated to the armature is in turn translated to the compressor drive shaft depends on the physical connection between the armature and shaft. One type of clutch accomplishes the torque transmission with an axially flexible, annular elastomer pad, the inner edge of which is fixed to a central hub bolted to the shaft and the outer edge of which is fixed to the armature. The flexible pad allows some torsional wind up at initial engagement, and serves to control the incidence of resonant frequency. A good example of such an elastomer clutch is disclosed in coassigned U.S. Pat. No. 5,195,625. The concentricity of the armature relative to the central shaft and hub is entirely dependent on the radial integrity of the elastomer pad, since it is the only connection therebetween. Therefore, the patent just noted discloses cylindrical rings embedded in the elastomer pad to increase its radial stiffness and thereby keep it more nearly concentric to the shaft during operation.